Positron emission tomography (PET) is a nuclear medicine imaging modality based on the measurement of positron emission from radio-labelled tracer molecules. These positron-emitting radionuclides (radiotracers) allow biologic processes to be measured and partial or whole body images to be obtained which demonstrate sites of radiotracer accumulation. The system detects pairs of gamma rays emitted indirectly by the tracer molecule, which is introduced into the body on a biologically active molecule. Two-dimensional or three-dimensional images of tracer concentration within the body are then constructed by computer analysis.
Fludeoxyglucose (18F), or fludeoxyglucose F 18, also commonly called fluorodeoxyglucose and abbreviated [18F]FDG, 18F-FDG or FDG, is a radiopharmaceutical used in PET. Chemically, it is 2-deoxy-2-(18F)fluoro-D-glucose, a glucose analog, with the positron-emitting radioactive isotope fluorine-18 substituted for the normal hydroxyl group at the 2′ position in the glucose molecule.
If the biologically active molecule selected for PET is FDG, the concentrations of tracer imaged will indicate tissue metabolic activity as it corresponds to the regional glucose uptake. PET is both a medical and research tool. It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), and for clinical diagnosis of certain diffuse brain diseases such as those causing various types of dementias. PET is also an important research tool to map human brain and heart function, and support drug development. PET is also used in pre-clinical studies using animals, where it allows repeated investigations in the same subjects. This is particularly valuable in cancer research, as it results in an increase in the statistical power of the data (subjects can act as their own control) and substantially reduces the numbers of animals required for a given study.
Other imaging modalities include x-ray computed tomography (CT), magnetic resonance imaging (MM) and functional magnetic resonance imaging (fMRI), ultrasound, and single-photon emission computed tomography (SPECT).